1. Field of the Invention
This invention relates to devices and methods for controlling the delivery and the delivery direction of an aerosol, and particularly to a method and apparatus for induced aerosol flow in an electrohydrodynamic (EHD) sprayer.
2. Background
The use of electrohydrodynamic (EHD) apparatus to produce aerosols is well known. Recently, we have recognized that EHD devices are extremely useful to produce and deliver aerosols of therapeutic products.
In typical EHE) devices fluid delivery means deliver fluid to be aerosolized to a nozzle maintained at high electric potential. One type of nozzle used in EHD devices is a capillary tube that is capable of conducting electricity. An electric potential is placed on the capillary tube which charges the fluid contents such that as the fluid emerges from the tip or end of the capillary tube a so-called Taylor cone is formed. This cone shape results from a balance of the forces of electric charge on the fluid and the fluid's own surface tension. Desirably, the charge on the fluid overcomes the surface tension and at the tip of the Taylor cone, a thin jet of fluid forms and subsequently and rapidly separates a short distance beyond the tip into an aerosol. Studies have shown that this aerosol (often described as a soft cloud) has a fairly uniform droplet size and a high velocity leaving the tip but that it quickly decelerates to a very low velocity a short distance beyond the tip.
EHD sprayers produce charged droplets at the tip of the nozzle. Depending on the use, these charged droplets can be partially or fully neutralized (with a reference or discharge electrode in the sprayer device) or not. The typical applications for an EHD sprayer without means for discharging or means for partially discharging an aerosol would include a paint sprayer or insecticide sprayer. These types of sprayers may be preferred since the aerosol would have a residual electric charge as it leaves the sprayer so that the droplets would be attracted to and tightly adhere to the surface being coated. However, with EHD apparatus used to deliver therapeutic aerosols, it is preferred that the aerosol be completely electrically neutralized prior to inhalation by the user to permit the aerosol to reach the pulmonary areas where the particular therapeutic formulation is most effective.
The preferred orientation of EHD sprayers is with the nozzle vertical and located above the object to receive the aerosol. This nozzle orientation eliminates, for practical purposes, the problems associated with the fluid dispensed from the nozzle tip collecting on or wicking up the outside of the capillary tube and associated fluid delivery means. If the fluid flows up the outside of the nozzle from the tip, it is no longer available to be sprayed and represents a loss in efficiency of the device. Moreover, fluid on the outside surfaces of the capillary tube may accumulate and suddenly flow back to the tip where it may disrupt the Taylor cone. These disruptions and any other disruptions of the Taylor cone may result in a large variation in the size and size distribution of the aerosol droplets which is particularly undesirable in pulmonary drug delivery.
When administering pharmaceuticals to a patient these limitations on orientation of the EHD apparatus result in either the patients having to tilt their head backwards or to lie on their back when the aerosol is delivered on axis with the nozzle. Alternatively, the EHD apparatus can deliver the aerosol vertically on axis with the nozzle and an elbow means can change the direction of aerosol flow to deliver the aerosol nearly horizontally. With this change in direction of the aerosol, there often is an appreciable loss in the quantity of the aerosol. The loss in quantity is a result of the fluid impacting and depositing on the walls of the delivery device, particularly in the vicinity of the elbow, instead of reaching the patient. One device for reducing disruptions of the Taylor cone and for reducing the loss in quantity of fluid impacting the walls is described in a co-owned U.S. patent application filed of even date herewith and entitled "High Mass Transfer EHD Aerosol Sprayer", which application is hereby incorporated by reference. Therefore, an EHD aerosol sprayer is needed where the aerosol delivery direction can be controlled and wherein the Taylor cone can be stabilized to prevent disruption. Of particular need, is an EHD aerosol sprayer that can spray substantially horizontally and deliver the aerosol without appreciable wetting of the delivery device.